When reading thru the power supply schematic of a sharp brand television, I noticed the link between the low voltage ground and the mains rectified DC ground.
What is the purpose of this? Don't we achieve best galvanic isolation if such link is not there?
It's quite common to put Y rated capacitors across the transformer in a switch mode power supply such as here: -
Without the capacitor, the secondary would/could be, as a whole entity, oscillating up and down at about 50% of the AC voltage of the primary voltage and this creates a lot of EMI on the output wires. It will do this because of the capacitive coupling between primary and secondary of the transformer and, the relatively high switching frequencies involved.
So, to obtain EMC approvals, a lot of companies use a capacitor in this position.
In the circuit above best EMI attenuation is acheived by connecting the isolated return conductor (via the Y capacitor) directly to the rectified positive rail but, for a different design this could equally work well with the rectified ground.
That link is two capacitors in series, so there is no DC current path through it. However, it will allow high-frequency signals straight through, which provides a path to ground for noise and EMI in the system.
The use of two capacitors and the Y-rating on the capacitors is there to prevent the isolation failing if one of the capacitors fails.
This link shows dozens of examples of how to design a high quality professional TV for safety on a single Cu layer board for 3kV no damage and fused above this. See if you can spot why they were done this way. ( in grey)Schematic
Galvanic isolation is achieved for DC but at 120 Hz or 100 Hz the CM leakage current is permitted to a SAFE limit of 500uA worst case, while it's main purpose is shunting high dV/dt noise to a lower common mode impedance to reduce emanations and crosstalk when using isolated ac-dc SMPS with many external HDMI connections.
These are important safety caps that are line to ground rated for voltage called Y caps. X caps are line to line.
When does capacitor reliability become critical to safety?
Line filter capacitors are classified either as X-capacitors or Y-capacitors. X-capacitors are connected between line and neutral, to protect against differential mode interference. Their failure does not create conditions for dangerous electric shock, although it can create a fire risk. However Y-capacitors are designed to filter out common-mode noise, and are connected between line and chassis; if they short-circuit, they create a risk of shock to the user.
How are Y-Capacitors designed and deployed to ensure safety?
Y-capacitors are designed to enhanced electrical and mechanical reliability standards. Capacitance values are also limited to reduce the current passing through the capacitor when AC voltage is applied, and reduce the energy stored to a safe limit when DC voltage is applied. Capacitors must be tested to applicable standards to qualify them for use as Y-capacitors.
Which European standards are applicable?
The EN 132400 Standard was issued on 26 June, 1995 replacing all the European National Standards in force up to that date. This was identical to the International Standard IEC 60384-14 2nd Edition 1993. Since then, to make CENELEC and IEC standards identical in name as well as specification, the European Standard EN 132400 has been superseded by EN 60384-14 that is identical to the International Standard IEC 60384-14. Any European national body can issue approvals, with validity recognized by the bodies of all the other CENELEC member countries with no need to repeat the tests.
http://powerblog.vicorpower.com/2013/06/what-are-y-capacitors/
You can learn much more about Electronic Design by studying the service manual schematics and layouts of high end quality companies like Sharp and in this case your TV service manual.
Note how the reference designators of all,X caps are CXxxxx and Y caps are CYxxxx and that there are many for different purposes.
The ones you show, are not (as might be thinking) for the input rectifier Line "Pi" LC filter Y caps , but rather the DC chassis to AC Common mode ground. See bottom in column 15.
